1. Field of the Invention
The present invention relates to a novel lighting circuit for a discharge lamp, which can shorten the time required for the flux of light to become stable when the discharge lamp is turned on again immediately after deactivation.
2. Description of the Related Art
Recently, compact metal halide lamps are receiving greater attention as a light source to take the place of an incandescent lamp. A known lighting circuit for a vehicular metal halide lamp is designed to use a DC power supply in such a way that a DC input voltage after being boosted is converted to an AC voltage of a rectangular waveform which is in turn applied to the metal halide lamp.
Because of slow starting of such a metal halide lamp from the cold state (hereinafter called "cold start"), one known control method temporarily supplies power greater than the rated power at the beginning of the lighting of the metal halide lamp to accelerate the lighting of the amp, thus improving the rising characteristic of the flux of light.
FIG. 13 exemplifies a lamp control curve, showing a lamp voltage (VL) on the horizontal scale and a lamp current (IL) on the vertical scale.
A control curve a can be separated into three areas with respect to VL. In an area Aa where VL is minimum, IL becomes maximum, whereas in an area Ac where VL is large, approximated constant power control with nearly the rated power is executed along the control line obtained by the linear approximation of a constant power curve. In the diagram, "Ps" is the operational point where the voltage and current indicate the rated values.
In an area Ab located between the areas Aa and Ac, control is so performed as to decrease IL as VL increases.
In a case where a metal halide lamp is reactivated immediately after deactivation (hereinafter called "hot start"), the above-described lighting control suffers a little longer time for the flux of light from the metal halide lamp to reach the steady level because the lighting acceleration control is not executed or is not effective in the area Ab.
As shown in FIG. 13, in the transition from point Ph on the constant power control line to point Ps at the hot start, the lighting acceleration control in the area Ab is not effected, and in the transition from point Pi on the control line in the area Ab to point Ps, the lighting acceleration control in the area Ab is inadequate. As indicated by a curve b in FIG. 14, therefore, a change in the flux of light L of the metal halide lamp from the point of rising to the point of reaching the steady level is gentle so that it takes time for the flux of light to reach the steady level. FIG. 14 presents an example of a change in the flux of light at the hot start, showing the time t on the horizontal scale and the flux of light L on the vertical scale (whose steady level is indicated by 100%).